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United States Patent |
5,217,944
|
Tournier
|
June 8, 1993
|
Crystal making method
Abstract
A method for preparing a textured polycrystalline material having, in the
crystalline state, a magnetic anisotropy, comprises the following steps:
preparing a compound such that, after being molten and solidified, it
provides substantially only said material and that, at the melting
temperature, crystallites of said material exist;
slowly heating close to the melting temperature, up to some degrees above
said temperautre, so that it remains crystallites of said material in a
liquid phase;
slowly cooling close to the melting temperature, up to solidification; and
applying, at least from the time at which the material begins to enter the
liquid state up to the time at which it is fully solidified, a magnetic
field having a sufficient strength to preferentially orient, despite the
thermal agitation energy, crystallites having a sufficient size for
constituting crystallization seeds.
Inventors:
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Tournier; Robert (Bilieu, FR)
|
Assignee:
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Centre National de la Recherche Scientifique (Paris, FR)
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Appl. No.:
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738510 |
Filed:
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August 1, 1991 |
Foreign Application Priority Data
Current U.S. Class: |
505/400; 117/37; 117/73; 505/450; 505/729 |
Intern'l Class: |
C30B 011/04 |
Field of Search: |
156/600,621,624
505/1,729
|
References Cited
U.S. Patent Documents
4939121 | Jul., 1990 | Rybka | 505/1.
|
4956339 | Sep., 1990 | Yamazaki | 505/1.
|
4990493 | Feb., 1991 | Lay | 505/1.
|
5039653 | Aug., 1991 | Jackson et al. | 505/1.
|
Foreign Patent Documents |
0284534 | Oct., 1988 | EP.
| |
64-81127 | Mar., 1989 | JP | 505/727.
|
1-108156 | Apr., 1989 | JP.
| |
2-7309 | Jan., 1990 | JP | 505/727.
|
Other References
"Control of Crystallization Processes by Means of Magnetic Fields", Journal
of Crystal Growth, vol. 52, 1981, pp. 524-529, by Mikelson et al.
N.T.I.S. Technical Notes, No. 4, 1986, NASA Technical Brief "Damping Melt
Convention with a Magnetic Field" 1986.
|
Primary Examiner: Kunemund; Robert
Attorney, Agent or Firm: Lowe, Price, LeBlanc & Becker
Claims
I claim:
1. A method for preparing a textured polycrystalline material having, in
the crystalline state, a magnetic anisotropy, comprising the following
steps:
preparing a compound such that, after being molten and solidified, said
compound provides substantially only said textured polycrystalline
material and that, at a melting temperature, crystallites of said material
exit;
slowly heating said compound from below said melting temperature, up to a
second temperature slightly above said melting temperature at which
crystallites of said material remain in a liquid phase;
slowly cooling said compound to cause solidification thereof; and
applying, at least from a time at which the material begins to enter the
liquid state up to a time at which it is fully solidified, a magnetic
field having a sufficient strength to preferentially orient, despite the
thermal agitation energy, crystallites of said material having a
sufficient size for constituting crystallization seeds.
2. A method according to claim 1, further comprising maintaining said
compound at a constant temperature level for a period of time between the
heating and cooling steps.
3. A method according to claim 1, wherein said material is a high
temperature superconductor.
4. A method according to claim 3, wherein said material is a superconductor
of the RBaCuO type where R designates a rare earth element.
5. A method according to claim 4, for preparing YBa.sub.2 Cu.sub.3 O.sub.7,
wherein:
said compound is a stoichiometric mixture of Y.sub.2 BaCuO.sub.5,
BaCuO.sub.2 and CuO in the form of a pressed powder;
the heating is carried out between 700.degree. and 1040.degree. C. with a
temperature gradient of about 100.degree. C. per hour, the compound being
maintained at 1040.degree. C. for about two hours; and
the cooling is carried out with a gradient of about 20.degree. C. per hour.
6. A method according to claims 5, wherein said magnetic field is applied
to said compound when said compound is heated to about 700.degree. C. and
the application of said magnetic field to said compound is maintained as
long as the temperature of said compound remains above about 700.degree.
C.
Description
BACKGROUND OF THE INVENTION
The invention relates to a method for obtaining a crystallized material in
presence of a magnetic field. It generally applies to any material
presenting a magnetic anisotropy in the crystal state although it is more
specifically disclosed hereinafter in connection with the manufacturing of
high temperature superconducting materials such as materials of the RBaCuO
family where R is a rare earth element such as yttrium.
Many authors have also indicated the possibility and the advantages of
applying a magnetic field during sintering for improving the orientation
or the size of crystalline grains to be obtained.
A part of the prior art documents relates to crystallization improvement
when the material to be obtained is placed in an non-homogeneous medium.
An example of such a prior art document is article of A. E. Mikelson and
al. published in the Journal of Crystal Growth, 52, 1981, pages 524-529.
In this article, it is taught that, if a magnetic field of about one tesla
is applied to a cadmium-zinc alloy in the molten state, one obtains
cadmium-zinc dendrites in an alloy having a different composition. These
dendrites are oriented along the field lines.
In other articles, which more particularly deal with obtaining high
temperature superconductors, it is suggested to make a superconductive
ceramic by sintering and pressing, then annealing in presence of a field
for improving the crystalline structure of the ceramic. So, in the
European Patent Application 0 284 534, the first example relates to YbaCuO
manufacturing and three successive annealing are disclosed, the first
between 500.degree. and 1200.degree. C., for example 700.degree. C., the
second between 500.degree. and 1200.degree. C., for example 900.degree.
C., and the third between 600.degree. and 1200.degree. C., for example
800.degree. C. So, in all the specific examples disclosed, although the
temperature range indicated by the applicant reaches the melting
temperature of YBaCuO (close to 1150.degree. to 1200.degree. C.), it is
taught to stay below this temperature and no difference is made between
what occurs below and above the melting temperature.
The present invention teaches a method comprising applying a magnetic field
to a material during the crystallization or recrystallization, wherein
this magnetic field is applied while the material is in a liquid phase and
comprises crystallites liable to constitute crystallization seeds when
cooling.
It will be shown that the implementation of such a method permits to obtain
a polycrystalline composition wherein the grains are much better oriented
than with the conventional methods and in particular when the field
application is not made while the substance is in a liquid phase.
Before explaining in greater detail the invention, some general law of
magnetism will be recalled.
Magnetic materials have a magnetic susceptibility .chi. which is generally
anisotropic. For example, there are materials that have an axis of easy
magnetization, hereinafter called axis c, the two other axes being axes a
and b. Thus, if .chi. is the magnetic susceptibility, the difference in
magnetic suspectibility between the axis of easy magnetization (c) and the
hard magnetization directions (a and b), is:
.DELTA..sub..chi. =.sub..chi.c -.sub..chi.ab
If a magnetic field B is applied, particles tend to be oriented according
to their axis of easy magnetization and an energy gain .DELTA.E is
produced with respect to the case of a material with a random distribution
of the magnetic axes:
.DELTA.E=V.B.sup.2..DELTA..sub..chi. /2.sub..mu.0
where V is the volume considered and .mu..sub.0 =4.pi..10.sup.-7 in
international units (I.U.).
If it is desired to orientate a magnetic material in a field, this energy
gain .DELTA.E must be substantially higher than the energy associated with
the thermal agitation, namely, kT, where T is the absolute temperature and
k the Boltzmann's constant.
The result of this comparison gives the definition of volumes or elementary
domains liable to be satisfactorily oriented. For example, for a YBa.sub.2
Cu.sub.3 O.sub.7 grain of 1 .mu.m.sup.3, which constitutes a high
temperature superconductor, .DELTA..sub..chi. will be about 10.sup.-5 I.U.
which gives .DELTA.E/kT=10.sup.4 at T=1500.degree. K. and for B=5 teslas,
that is, .DELTA.E.gtoreq.kT. But, .DELTA.E/kT is equal to 10 only if the
grain size decreases to 10.sup.-3 .mu.m.sup.3.
The simple case of an uniaxial anisotropy will be considered here. However,
it is known that some magnetic materials may have several equivalent axes
of easy magnetization and even an easy magnetization plane. This magnetic
anisotropy may be very high when the material is magnetically ordered,
particularly when it is ferromagnetic. In the paramagnetic state, the
magnetic anisotropy is low but often sufficient for alignment under a
magnetic field.
It will be reminded that the magnetic susceptibility .chi. varies with the
inverse of the square temperature (1/T.sup.2), that is .chi. decreases
quickly when T increases.
SUMMARY OF THE INVENTION
The invention provides for a method for preparing a textured
polycrystalline material having, in the crystalline state, a magnetic
anisotropy, comprising the following steps: preparing a compound such
that, after being molten and solidified, it provides substantially only
said material and that, at the melting temperature, crystallites of said
material exist; slowly heating close to the melting temperature, up to
some degrees above said temperature, so that it remains crystallites of
said material in a liquid phase; slowly cooling close to the melting
temperature, up to solidification; and applying, at least from the time at
which the material begins to enter the liquid state up to the time at
which it is fully solidified, a magnetic field having a sufficient
strength to preferentially orient, despite the thermal agitation energy,
crystallites having a sufficient size for constituting crystallization
seeds.
Between the heating and cooling steps, a step corresponding to a constant
temperature level may be provided for.
According to an embodiment of the invention, said material is a high
temperature superconductor.
According to an embodiment of the invention, said material is a
superconductor of the RBaCuO type where R designates a rare earth element.
The invention, applies to the preparation of YBa.sub.2 Cu.sub.3 O.sub.7.
Then, said compound is a stoichiometric mixture of Y.sub.2 BaCuO.sub.5,
BaCuO.sub.2 and CuO in the form of a pressed powder; the heating is
carried out between 700.degree. and 1040.degree. C. with a temperature
gradient of about 100.degree. C. per hour, the compound being maintained
at 1040.degree. C. for about two hours; and the cooling is carried out
with a gradient of about 20.degree. C. per hour. In apreferred embodiment,
the magnetic field is applied as soon as the compound attains the
temperature of about 700.degree. C. and as long as it is above said
temperature.
It will be appreciated that, according to a basic aspect of the invention,
the heating step is carried out so that, at the beginning of the cooling
step, there are crystallites of the material to be obtained having sizes
adapted to constitute crystallization seeds.
This implies that the temperature at which the material is set above the
melting temperature is not too high so that the sizes of the seeds do not
become too small so that the magnetic field is operative on to those
seeds. In other words, overheating has to be avoided.
This also implies that such seeds are initially present. Accordingly, in
the process wherein the starting composition comprises a stoichiometric
mixture of precursors of the material to be obtained, the temperature
increases before the melting itself must be made with a sufficiently low
temperature gradient so that such seeds are created.
This also implies, for example in the case where the starting material is
already crystallized and has to be recrystallized, that the temperature
increase must be high enough so that the crystallites having a too large
size "melt" because, despite the presence of a liquid phase, such too big
crystallites could be disturbed by their neighbours and prevent obtaining
a satisfactory orientation.
Once a thermal process has been provided, so that, in the molten state,
crystallites having suitable sizes are present, it will be necessary, in
accordance with the above formulae, to choose a sufficient magnetic field
strength for orienting such seeds despite thermal agitation phenomenons,
that is the relation .DELTA.E.gtoreq.kT has to be satisfied.
The invention will be disclosed hereinafter in connection with specific
embodiments, but it will be understood that those skilled in the art, in
view of the above criterions, will be able to implement the invention with
any anisotropic magnetic material to be oriented and textured during a
crystallization or recrystallization step.
The following description, for better emphasizing the conditions to be met,
will indicate, on the one hand, results of experiments not strictly
implementing the inventive methods and being unsuccessful, and, on the
other hand, an example of an experience implementing the invention and
providing a suitable grain texturation or orientation of a polycrystalline
substance.
BRIEF DESCRIPTION OF THE DRAWINGS
This description will be made in connection with the attached drawings
wherein:
FIGS. 1 and 2 illustrate magnetization curves as a function of the field
for high temperature superconductive materials in an heterogeneous medium;
FIG. 3 shows a temperature diagram corresponding to the method according to
the invention applied to an YBaCuO type material;
FIG. 4 shows a magnetization curve as a function of the field for an YBaCuO
type material having been submitted to the inventive process; and
FIG. 5 shows a magnetization curve as a function of the field for an YBaCuO
type material that has been placed at a temperature lower than its melting
temperature.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before disclosing the invention, preliminary experiments made by the
inventors for illustrating the possibility of texturing a substance of the
RBaCuO type in a molten heterogeneous medium in the presence of a magnetic
field will be disclosed. Although, the considered materials are
conventionally called here RBaCuO or RBa.sub.2 Cu.sub.3 O.sub.7, the exact
formula is RBa.sub.2 Cu.sub.3 O.sub.7-.delta..
The applicant has submitted various powders of the RBa.sub.2 Cu.sub.3
O.sub.7 type (where R designate a rare earth) mixed with a powder of
silver oxide (Ag.sub.2 O), milled and sintered, to a thermal cycle up to a
temperature close to the melting point of RBa.sub.2 Cu.sub.3 O.sub.7. At
such temperatures, Ag.sub.2 O is a liquid. After a thermal cycle up to
about 1020.degree. C. and a cooling at a rate of some tens of degrees per
hour, a texturation of the compound has been obtained, that is the
RBa.sub.2 Cu.sub.3 O.sub.7 grains contained in the compound are oriented
along a preferential direction. More particularly, if the rare earth is
yttrium, holmium, samarium or europium, the c axis of the crystalline
structure of the crystallites is oriented parallel to the applied field
because it is the axis of easy magnetization for those crystallites.
However, if it is erbium, the c axis is oriented perpendicularly to the
applied field because it is then an axis of hard magnetization while the
(a, b) plane is a plane of easy magnetization. The occurrence of this
anisotropy is illustrated by FIGS. 1 and 2 which respectively correspond
to a mixture of YBa.sub.2 Cu.sub.3 O.sub.7 and Ag.sub.2 O and a mixture of
ErBa.sub.2 Cu.sub.3 O.sub.7 and Ag.sub.2 O. The curves of FIGS. 1 and 2
illustrate the magnetization M at 4.degree. K as a function of the applied
field H. In each figure, curve 1 corresponds to a measurement field
perpendicular to the applied field H.sub.m during the cycle and curve 2 to
a measurement field parallel to the field H.sub.m applied during the
cycle. In those various cases, the fields H.sub.m were of some teslas.
Those preliminary experiments show that, despite the decrease of the
magnetic susceptibility .chi. with an increase of the temperature, it is
possible for reasonable values of the field to cause an orientation of the
RBa.sub.2 Cu.sub.3 O.sub.7 crystallites in a liquid medium at temperatures
as high as 1020.degree. C.
In an example, YBa.sub.2 Cu.sub.3 O.sub.7 has been prepared from three
precursors Y.sub.2 BaCuO.sub.5, BaCuO.sub.2 and CuO in stoichiometric
proportions. It will be shown that, for permitting an orientation in a
liquid phase, a suitable thermal sequence has to be used so that
crystallites of the material to be obtained (YBa.sub.2 Cu.sub.3 O.sub.7)
of a suitable size exist in the liquid phase.
The above indicated mixture, initially sintered, is used and the
temperature is raised up to melting (1040.degree. C.). This temperature
increase has to be sufficiently slow so that the precursors react in a
solid phase before melting to form crystallites (this normally occurs
during a sintering operation). For example, the temperature increase rate
can be about 100.degree. C. per hour between 700.degree. and 1040.degree.
C. Then, as shown in FIG. 3, the temperature is maintained at a constant
level of 1040.degree. C. during some hours, for example two hours. During
the final phase of the temperature increase and during the constant
temperature phase, a magnetic field of some teslas, for example 7T, is
applied. Then, the temperature is slowly decreased, at a rate of some
degrees per hour, for example 20.degree. C. per hour. The magnetic field
is maintained up to solidification and may be maintained below. So, as
shown in FIG. 4, an YBa.sub.2 Cu.sub.3 O.sub.7 ceramic having a quasi
total anisotropy is obtained, that is a material the crystalline grains of
which are oriented parallel to each other, the c axis being parallel to
the magnetic field applied during the high temperature phase. The results
indicated in FIG. 4 are clearly confirmed by X-ray analysis.
A similar experiment wherein the maximum temperature was 1050.degree. C.
and the field 5T produced an oriented YBa.sub.2 Cu.sub.3 O.sub.7 ceramic
(orientation slightly perturbed by non-homogeneities of the temperature in
the crucible during the cooling) and with critical currents four times
higher than in the former example.
Then, various experiences have been made by the inventors to show the
limits of the process.
FIG. 5 illustrates the result of an experiment made in the same conditions
as in FIG. 3 but wherein the maximum temperature was 1020.degree. C. The
obtained YBa.sub.2 Cu.sub.3 O.sub.7 had a low anisotropy because the
liquid state had not been clearly attained.
With a constant temperature level of 1020.degree. C. maintained during 24
hours, no X-ray detectable result is obtained.
A constant temperature level of 1030.degree. C. also gave negative results.
In another experiment, the same cycle as the one of FIG. 3 has been
conducted but the temperature increase rate was 200.degree. C. per hour
instead of 100.degree. C. per hour and the maximum temperature was
1050.degree. C. Then, the obtained YBa.sub.2 Cu.sub.3 O.sub.7 was not
textured in the direction of the magnetic field. In this experiment, the
crystallites had not sufficient time to be formed in the solid phase at a
sufficient size and, due to the high temperature of the liquid, the
crystallites have been "dissolved" and had attained too small sizes to be
usable as crystallization seeds.
Another experiment has been made wherein the maximum temperature was about
1055.degree. C. Again, the obtained product did not exhibit an anisotropy
associated with the presence of the magnetic field. In this case, due to
the high temperature of the liquid phase (overheating), the crystallites
had been, as formerly, "dissolved".
Consistent experiments have been made when starting from chemical
precursors of HoBa.sub.2 Cu.sub.3 O.sub.7 raised at their melting
temperature.
As a conclusion, at least two conditions have to be taken into account for
implementing the method according to the invention. The first condition is
that seeds have to exist when the liquid phase occurs. The second
condition is that such seeds have not to be eliminated by a too high
temperature or a too high duration of the processing in the liquid phase.
Additionally, the applicant made experiments in accordance with the prior
art techniques wherein the magnetic field is applied below the liquid
phase temperature. Those experiments have not given any result as to a
clear variation of the anisotropy of the obtained ceramic. At best,
improvements of some percent (10-15%) (and not an anisotropy of
substantially 100% as the one obtained by the present invention) have been
observed.
The above discloses the results of experiments made by the applicant,
mainly on high temperature superconductors of the RBa.sub.2 Cu.sub.3
O.sub.7 type, and more particularly experiments wherein this high
temperature superconductor was obtained from precursors in a
stoichiometric mixture. The invention also applies when the starting
material is directly the final material to be oriented if this material is
heated over its melting temperature which is not the same as the one of
the precursor mixture. For example, for YBa.sub.2 Cu.sub.3 O.sub.7, the
melting temperature is higher than 1050.degree. C. instead of 1040.degree.
C. for the precursors mixture. The melting does not produce a liquid
without solid seeds and, up to temperatures slightly higher than the
melting temperature (some degrees, up to 10 to 15 degrees), crystallites
liable to form seeds remain in the liquid and permit to implement the
invention, or such seeds grow during the solidification process.
Additionally, other materials than the one indicated in the above examples
can be textured (crystallographically oriented) by the method according to
the invention in as much as such materials exhibit a magnetic anisotropy.
Additionally, it is known that, to facilitate the solidification of a
material according to its preferential growth axis or plane, it is
desirable to apply, during the cooling, a temperature gradient in the
direction of this growth axis or plane. Particularly, this means that,
during the solidification of a material, a temperature gradient can be
applied along a direction corresponding to the growth direction already
determined by the magnetic field applied according to the invention.
Thus, if we consider for example a rod or a wire of YBa.sub.2 Cu.sub.3
O.sub.7 comprising a molten zone submitted to a magnetic field transverse
to the rod direction, the longitudinal temperature gradient favorizes the
texturation along the rod or wire.
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